This application discloses an acidic crystalline catalyst which is surface inactivated by exposure to aqueous ammonia borane solution. The application further discloses uses for such catalysts, including a process for producing high molecular weight hydrocarbons from a lower olefin feedstock.
Recent work in the field of olefin upgrading has resulted in a catalytic process for converting lower olefins to heavier hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Such a technique has been developed by Garwood, et al, as disclosed in European Patent Application No. 83301391.5, published Sept. 29, 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640; 4,227,992; and 4,547,613 Garwood, et al disclose operating conditions for a process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10.sup.+ aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of non-reactive paraffins and the like being acceptable components.
Several techniques may be used to increase the relative ratio of intra-crystalline acid sites to surface active sites in acidic porous crystalline materials. This ratio tends to increase with crystal size due to geometric relationship between volume and superficial surface area. Deposition of carbonaceous materials by coke formation can also shift the effective ratio, as disclosed in U.S. Pat. No. 4,547,613.
It is known to use certain basic materials to deactivate the Bronsted acid sites on the surface of aluminosilicate catalysts. U.S. Pat. No. 4,520,221 and U.S. Pat. No. 4,568,786, Chen, et al., which are expressly incorporated herein disclose bulky amines, such as di-tert-butyl pyridine, as such basic materials.
U.S. Pat. No. 4,870,038 to Page et al. discloses olefin oligomerization using a zeolitic catalyst (ZSM-23) wherein the zeolite surface is rendered substantially inactive for acidic reactions by neutralizing with a bulky pyridine compound, e.g., 2,4,6-collidine.
U.S. Pat. No. 5,012,029 to Han et al. discloses a method for the selective oxidation of methane to liquid hydrocarbons over zeolitic catalyst, e.g., ZSM-5 or ZSM-23, by introducing a reaction modifier, e.g., borane (column 8, line 34), to the feed, which reduces selectivity to methanol.
U.S. Pat. No. 4,300,012 to Tu et al. discloses an alkylaromatic transalkylation catalyst whose selectivity is enhanced by treating a mordenite alumina with an aqueous ammoniacal solution, calcining, and then contacting with an aqueous solution containing a boron salt.
U.S. Pat. No. 4,613,720 to Kukes et al. teaches olefin oligomerization to C.sub.5 to C.sub.12 hydrocarbons over aluminosilicates having a silica to alumina ratio of less than 12 which is prepared by depositing a boron containing species on the surface region. Boranes are disclosed as a source of suitable boron containing species. The addition of boron reduces the formation of coke on the surface region of the catalyst at low exposures and enhances yields of light monoolefins.
U.S. Pat. No. 4,752,596 to Bergna et al. discloses an acidic zeolite such as chabazite, erionite, ZK-5 or rho which is used in the production of dimethylamine from methanol and ammonia. The zeolite is modified by treatment with, inter alia, boron compounds, e.g. boranes (column 13, line 27).
U.S. Pat. No. 4,806,512 to Elvin teaches the treatment of crystalline materials such as zeolites with an aqueous reductive wash medium, e.g., borane, followed by treatment with an oxidative wash medium in order to improve catalytic properties.
Shape-selective oligomerization, as it applies to the conversion of C.sub.2 -C.sub.10 olefins over ZSM-5, may produce higher olefins up to C.sub.30 and higher. As reported by Garwood in "Intrazeolite Chemistry 23", (Amer. Chem. Soc., 1983), reaction conditions favoring higher molecular weight product are low temperature (200.degree.-260.degree. C.), elevated pressure (about 2000 kPa or greater), and long contact time (less than 1 WHSV). The reaction under these conditions proceeds through the acid-catalyzed steps of (1) oligomerization, (2) isomerization-cracking to a mixture of intermediate carbon number olefins, and (3) interpolymerization to give a continuous boiling product containing all carbon numbers. The channel systems of medium pore catalysts impose shape-selective constraints on the configuration of the large molecules, accounting for the differences with other catalysts.
The desired oligomerization-polymerization products include C.sub.10.sup.+ substantially linear aliphatic hydrocarbons. This catalytic path for propylene feed provides a long chain which generally has lower alkyl (e.g., methyl) substituents along the straight chain.
The final molecular configuration is influenced by the pore structure of the catalyst. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross-section of the chain limited by the dimension of the largest zeolite pore. Although emphasis is placed on the normal 1-alkenes as feedstocks, other lower olefins, such as 2-butene or isobutylene, are readily employed as starting materials due to rapid isomerization over the acidic zeolite catalysts. At conditions chosen to maximize heavy distillate and lubricant range products (C.sub.20.sup.+), the raw aliphatic product is essentially mono-olefinic. Overall branching is not extensive and may occur at spaced positions within the molecule.
The viscosity index of a hydrocarbon lube oil is related to its molecular configuration. Extensive branching in a molecule usually results in a low viscosity index. Similarly, extensive branching can be problematic in distillate fractions which are utilized as diesel fuels. Such branching increases auto ignition delay above optimal operating levels in diesel engines. Cetane number is a measurable quantity which varies inversely with the extent of branching in the components of a diesel fuel. It is believed that two modes of oligomerization/polymerization of olefins can take place over acidic zeolites, such as HZSM-5. One reaction sequence takes place at Bronsted acid sites inside the channels or pores, producing essentially linear materials. The other reaction sequence occurs on the outer surface, producing more branched material. By decreasing the surface acid activity of such zeolites, fewer highly branched products with low VI or low cetane number are obtained.